The current Long Term Evolution (LTE) standard only specifies uplink (UL) trans-missions from a single antenna at a time. This framework allows for Multi User Multi Input Multi Output (MU-MIMO) transmission schemes or spatial domain multiple access, i.e. simultaneous transmission of multiple mobile terminals on the same time-frequency resources. From a terminal perspective, this transmission mode is the same as the standard single-antenna UL transmission, only the base station is required to implement a special receiver.
In order to estimate the propagation channels between the various terminals and the base station used for MU-MIMO transmission schemes, mutually orthogonal reference signals are needed. In the LTE standard, these orthogonal reference signals r(α)(n) are defined in the frequency domain as shown in equation (1):r(α)(n)=ejαn× r(n),n=0,1, . . . ,MSCRS−1,  (1)with MSCRS=mNSCRB being the number of allocated subcarriers and r(n) being a cell specific base sequence.
Since α=2πnCS/12, the multiplication of the cell specific base sequence r(n) with the complex exponential function in the frequency domain results in a cyclic shift of nCS*N/12 samples of the time domain base sequence, wherein N is the size of the Inverse Discrete Fourier Transform (IDFT) used in the modulator and nCS is the cyclic shift parameter indicating the cyclic shift of a respective reference signal.
Equation (1) together with the definition of α reveals that in total twelve different cyclic shifts (nCS=0, 1, . . . , 11) exist, each resulting in a different reference signal r(α)(n), the different reference signals being orthogonal to each other.
However, in the LTE standard only a 3-bit field is specified to signal the cyclic shift parameter nCS and thus α. Since three bits can only be used for representing eight different values, the 3-bit field can only signal eight possible cyclic shifts and thus eight orthogonal reference signals.
The cyclic shift parameter nCS is determined according to equation (2):nCS=(nDMRS(1)+nDMRS(2)+nPRBS)mod 12,  (2)with nDMRS(1) and nPRBS being cell specific parameters and nDMRS(2) being signalled to the UE as part of the UL scheduling grant message. The table shown in FIG. 1 illustrates values of the parameter nDMRS(2) dependent on the signalled values in the 3-bit field. If, for example, the bit field [010] would be signalled, the parameter nDMRS(2) would be set to 3. If, on the other hand, the bit field [110] would be signalled, the parameter nDMRS(2) would be set to 9. As can be seen from the table shown in FIG. 1, only eight different values for nDMRS(2) can be selected. That is, only eight different values for the cyclic shift parameter nCS can be signalled and thus only eight different orthogonal reference signals can be obtained.
One of the targets in LTE-Advanced (LTE-A) is an UL peak data rate of 500 Mb/s. To achieve this data rate, the UL transmission scheme in LTE must be extended to support a wider bandwidth and Single User MIMO (SU-MIMO).
SU-MIMO uses multiple transmitter and receiver antennas at a single mobile terminal to improve the system performance and is a technology which employs the multiple antennas to coherently resolve more information than possible using a single antenna. In addition, SU-MIMO technology relies on multipath signals. Multipath signals are the reflected signals arriving at the receiver some time after the line of sight (LOS) signal transmission has been received. In a non-MIMO based network, multipath signals were perceived as interference degrading a receiver's ability to recover the message information from the signal. In contrast, SU-MIMO uses the diversity of the multipath signal to increase a receiver's ability to recover the message information from the signal.
As mentioned above, another ability the SU-MIMO technology provides is Spatial Multiplexing (SM) using multiple transmit and receive antennas. SM is a transmission technique to independently transmit separately encoded data signals, so-called streams or layers, from each of the multiple transmit antennas within one spectral channel of bandwidth at the same time. Therefore, the space dimension is reused, or multiplexed, more than one time.
SU-MIMO SM can be used with or without precoding. In precoded SU-MIMO SM, one stream is split into a first plurality of substreams, a second stream is split into a second plurality of substreams and so on. Each of the first plurality of substreams, each of the second plurality of substreams and so on can then be transmitted by one of a plurality of transmit antennas. In short, each of a plurality of streams is subdivided into a plurality of substreams, each of which being transmitted by one of a plurality of transmit antennas. Mathematically speaking, this relation can be expressed by equation (3), which shows a multiplication of a precoding matrix M with a column vector s:
                                          M                          _              _                                ×                      s            _                          =                              (                                                                                M                    11                                                                                        M                    12                                                                    …                                                                      M                                          1                      ⁢                                                                                          ⁢                      s                                                                                                                                        M                    21                                                                                        M                    22                                                                    ⋱                                                  ⋮                                                                              ⋮                                                  ⋱                                                  ⋱                                                  ⋮                                                                                                  M                                          a                      ⁢                                                                                          ⁢                      1                                                                                        …                                                  …                                                                      M                    as                                                                        )                    ×                      (                                                                                s                    1                                                                                                                    s                    2                                                                                                ⋮                                                                                                  s                    s                                                                        )                                              (        3        )            
In the matrix M, the number of rows a is equal to the number of transmit antennas and the number of columns s is equal to the number of streams to be transmitted by the transmit antennas. In order to perform multiplication of the matrix M with the vector s, the number of rows of the vector s have to equal the number of columns of the matrix M. Understandably, the number of transmit antennas can be arbitrarily chosen bearing in mind that the number of transmit antennas should be equal to or greater than the number of streams.
In non-precoded SU-MIMO, one stream is transmitted per antenna. That is, in the specific case of non-precoded SU-MIMO, equation (3) results in equation (4), where the matrix M is a diagonal matrix because one stream is not subdivided into substreams but allocated to one transmit antenna:
                                          M                          _              _                                ×                      s            _                          =                              (                                                                                M                    11                                                                    0                                                  …                                                  0                                                                              0                                                                      M                    22                                                                    ⋱                                                  ⋮                                                                              ⋮                                                  ⋱                                                  ⋱                                                  ⋮                                                                              0                                                  …                                                  …                                                                      M                    as                                                                        )                    ×                      (                                                                                s                    1                                                                                                                    s                    2                                                                                                ⋮                                                                                                  s                    s                                                                        )                                              (        4        )            
That is, SU-MIMO SM can significantly increase data throughput as the number of resolvable spatial data streams is increased. In precoded SM, a high rate stream is split into multiple lower rate substreams and each substream is transmitted from a different transmit antenna in the same frequency channel at the same time. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams, leading to an increase of the spectral efficiency, which is the information rate that can be transmitted over a given bandwidth (the number of bits per second and per Hz that can be transmitted over the wireless channel). In non-precoded SM, each spatial stream requires a discrete antenna at both the transmitter and the receiver. The spectral efficiency in non-precoded SM is increased by transmitting a plurality of streams from a plurality of transmit antennas at the same time. In both cases, precoded and non-precoded SM, the number of simultaneous data streams is limited by the minimum number of antennas in use on both sides of the link.
SM can be used with or without transmit channel knowledge. If SM with transmit channel knowledge should be used, reference signals are needed to estimate the respective transmission channels. Since SM requires multiple transmit and receiver antennas, multiple transmission channels are provided, wherein for estimation of each of these multiple transmission channels a dedicated reference signals is needed.
Thus, in case of SU-MIMO multiple reference signals need to be transmitted per mobile terminal. Signalling each required cyclic shift independently substantially increases overhead. Furthermore, using the conventional LTE signalling approach discussed above, only eight out of twelve theoretically possible cyclic shifts can be signalled.